What are the types of loft conversion?

There are four main types of loft conversion in the UK: Velux (rooflight), dormer, hip-to-gable and mansard. They run in that order for cost and disruption, from around £25,000 for a Velux conversion to £70,000 or more for a mansard, based on independent UK market research.

Which one suits your house is mostly decided by your roof, not your preferences: the shape it is now, the head height inside it, and what your street's planning context allows. All four are delivered through Beams' loft conversions in London service. Here's how the four compare and how to tell which one your home is a candidate for.

What is a velux (rooflight) loft conversion?

A Velux conversion keeps the existing roof shape and adds rooflight windows, insulation, flooring and a staircase, typically costing £25,000 to £40,000. It's the cheapest and least disruptive type, but only works where there's already generous head height.

Because the roofline doesn't change, most Velux conversions fall under permitted development and many don't trouble the planners at all. The trade-off is space: you keep the sloping ceilings, so the usable floor area is whatever the existing roof gives you. As a rule of thumb you need around 2.2m to 2.3m from the top of the existing joists to the ridge for the finished room to work.

Best for: bungalows and houses with steep, tall roofs; budgets under £40,000; homes in conservation areas where altering the roofline is difficult.

What is a dormer loft conversion?

A dormer conversion extends the roof outwards with a flat-roofed (or occasionally pitched) box, adding full-height space and vertical windows, typically costing £35,000 to £60,000. It's the most common loft conversion in the UK because it works on most house types.

The standard configuration is a rear dormer running much of the width of the roof, which turns a cramped triangle into a proper bedroom with room for an ensuite. Rear dormers usually fall within permitted development limits (40 cubic metres of added roof space for terraces, 50 for semis and detached homes), which is a big part of their popularity. If you're weighing up the window-only route against a dormer, the space gain is usually decisive; the cost of the box itself is covered in our dormer window cost guide.

Best for: terraced and semi-detached houses needing a double bedroom plus ensuite; projects that want to avoid a planning application.

What is a hip-to-gable loft conversion?

A hip-to-gable conversion rebuilds the sloping side of the roof (the hip) into a vertical wall (a gable), creating the internal volume a hipped roof lacks, typically costing £45,000 to £65,000. It's usually combined with a rear dormer for maximum space.

Semi-detached and end-of-terrace houses from the 1930s are the classic candidates, because their hipped roofs leave too little head height for a straight conversion. The hip-to-gable-plus-dormer combination is what most of those houses actually build. Detached houses with hips on both sides can convert both, at proportionally higher cost.

Best for: 1930s semis and end-of-terrace homes; anyone whose hipped roof makes a standard dormer conversion too tight.

What is a mansard loft conversion?

A mansard conversion rebuilds one or both roof slopes to a near-vertical 72-degree profile with a flat top, effectively adding a full extra storey, typically costing £55,000 to £80,000 or more. It creates the most space of any type and almost always needs planning permission.

Mansards are the London special: they suit period terraces, match a roofline pattern many streets already have, and maximise every cubic metre on plots where space is gold. The cost reflects what's really happening, which is closer to rebuilding the roof than converting it. On terraces, party wall agreements with both neighbours come as standard, and conservation area rules apply on many of the streets where mansards make most sense.

Best for: London terraces chasing maximum space; streets where mansards are already established; owners planning to stay long enough to justify the spend.

Which type of loft conversion should you choose?

The right loft conversion type follows from three checks: measure your existing head height, identify your roof shape, and confirm your planning context. Those three answers usually eliminate all but one or two options.

Type	Typical cost	Space gained	Planning permission
Velux / rooflight	£25,000 – £40,000	Low	Rarely needed
Dormer	£35,000 – £60,000	High	Usually permitted development
Hip-to-gable	£45,000 – £65,000	High	Often permitted development
Mansard	£55,000 – £80,000+	Maximum	Almost always needed

Head height under about 2.2m at the ridge pushes you toward the types that rebuild the roof; the exact numbers are covered in loft conversion head height requirements. A hipped roof points at hip-to-gable. A conservation area points away from mansards and toward rooflights. Terraced houses have their own quirks around party walls and stair positions, which our guide to converting a loft in a terraced house covers in detail, and every type carries the same building regulations backbone: a compliant staircase, fire escape route and insulation. For budgeting the project as a whole and how long it takes, see our loft conversion cost guide and loft conversion timelines.

Working out your type with Beams

Reading your own roof is the hard part: head heights, purlin positions, stair landings and planning context all interact. Beams handles that assessment as part of planning your project, then puts a detailed scope in front of up to three vetted London loft specialists to quote like for like, with fixed prices and milestone payments. Get a free estimate and find out which conversion your roof is actually built for.

Where these figures come from

The cost ranges in this guide are drawn from independent UK sources rather than Beams' own pricing: BCIS (the RICS Building Cost Information Service) build cost data, Office for National Statistics construction output data, Federation of Master Builders guidance, and government planning guidance on permitted development for roof extensions. Every conversion prices differently once the roof structure and staircase position are known.

Sources

BCIS (RICS Building Cost Information Service): https://bcis.co.uk
Office for National Statistics, construction statistics: https://www.ons.gov.uk/businessindustryandtrade/constructionindustry
Federation of Master Builders: https://www.fmb.org.uk
Planning Portal, loft conversions and permitted development: https://www.planningportal.co.uk
HM Government, Approved Documents (fire safety, staircases, insulation): https://www.gov.uk/government/collections/approved-documents

Ready to start planning your renovation?

A Beams planner can assess what's possible for your home, walk you through the process and give you a clear estimate - no obligation.